1. Field of the Invention
The present invention generally relates to a communication apparatus in which the transmission level of a modem can be adjusted, and more specifically relates to such a communication apparatus that detects a line current and a line voltage to calculate line impedance and adjusts the transmission level of the modem based on the calculated impedance.
2. Description of the Related Art
A two-wire type communication line such as a general public telephone line has line impedance, which varies according to the length from a central telephone office. The variance in the line impedance affects the line current. If the line impedance is high, sufficient current cannot flow and normal communications are obstructed, or attenuation in the communication line to a switchboard may be increased so as to degrade the communications quality.
When the communication lines are connected via a private branch exchange, the amount of line current may vary depending on characteristics of the exchange and the communications quality may vary accordingly.
There have been some proposals for avoiding communications quality degradation. One proposal is to detect a line current and adjust the impedance of a two-wire to four-wire converting circuit to suppress sidetone generation. Another proposal is to detect line impedance and inhibit the communications quality from degrading due to changes in the manner of supplying power to the subscriber lines. These proposals are disclosed in Japanese Laid-open Patent Applications No. 07-288577 and No. 08-233878.
In conventional network control units for controlling communication lines, the major elements are a primary side connected to the line and a secondary side isolatingly connected to the primary side via the conductance of a transformer. This structure generally employs an electric current sensor having a current detecting function provided by a Hall device and the like, to detect a current on the primary side.
Such a sensor detects the primary side line current by using a coil wound on a ferrite core, which senses a magnetic field generated by the line current and outputs a voltage proportional to the line current.
The thus detected voltage is an analog voltage, which cannot be computer-processed itself. Therefore the analog voltage is converted to a digital voltage for further processing. However, an A/D converter becomes necessary in addition to the current sensor and therefore additional mounting space is needed, resulting in high cost.
When detecting a line voltage under the condition that the line is not closed, a detecting circuit and an A/D converter for converting the detected amount of voltage are needed. These circuits should be placed on the primary side, and must be resistant to surges such as lightning surges. It is difficult to provide the actual mounting place for such circuits, resulting in higher cost.
With this background and the recent progress in semiconductor technology and isolation technology, semiconductor devices are being used to replace transformers.
A semiconductor DAA (Data Access Arrangement) can not only replace a conventional transformer but also perform other convenient functions, therefore resulting in higher performance, as shown in Japanese Laid-open Patent Application No. 06-098038.
However, it is difficult to find a mounting place for a line current detector or a line voltage detector, and the cost to manufacture them is high.
In addition, when a loss of the line current or the line voltage is detected, it is difficult to identify the cause of the failure.